This invention relates to a scanning electron microscope, or in particular, to a scanning electron microscope adapted for detecting the charged particles generated in low vacuum.
Some objects to be observed under the scanning electron microscope contain moisture and cannot be placed in high vacuum. To observe such objects, what is called a low-vacuum scanning electron microscope is used in which the atmosphere in the specimen chamber with a specimen arranged therein is maintained in low vacuum as compared with the high vacuum around the electron source.
In order to maintain the specimen chamber in low vacuum, a pressure limiting aperture to selectively keep the specimen chamber in low vacuum is arranged between the specimen chamber and the high vacuum area. This pressure limiting aperture has an opening sufficient to pass the electron beam.
With regard to the scanning electron microscope having the pressure limiting aperture, techniques for improving the detection efficiency of secondary electrons and reflected electrons are described in JP-A-9-320504 and JP-A-2002-75264.
According to JP-A-9-320504, the pressure limiting aperture is arranged on the principal plane of the objective lens, so that the trajectory of the reflected electrons diffused in the gas atmosphere is restricted on the electron beam optical axis by an objective lens magnetic field and detected by a reflected electron detector arranged in the neighborhood of the pressure limiting aperture.
According to JP-A-2002-75264, on the other hand, the secondary electrons are converged on the optical axis by the magnetic field of the objective lens, and after being passed through the pressure limiting aperture, detected by a secondary electron detector arranged on the pressure limiting aperture.
The scanning electron microscopes having an pressure limiting aperture disclosed in JP-A-9-320504 and JP-A-2002-75264 are both intended to detect the secondary electrons or the reflected electrons but not to detect the ions generated by the collision of the secondary electrons released from the specimen with gas molecules.
The lens described in JP-A-9-320504 is called the semi-in-lens or the lower-pole-open lens known to have a structure suitable for observation of a large specimen with high resolution in view of the fact that the specimen can be placed in the lens magnetic field and therefore the distance between the principal plane of the lens and the specimen (working distance (WD)) can be shortened accordingly.
These conventional techniques, however, pose the problem that the lens magnetic field leads the secondary electrons released from the specimen onto the objective lens, and therefore, the detection under the objective lens in the gas atmosphere is difficult.